The present invention relates generally to a transmitter of a radio system using burst transmission, and particularly to a ramp generator for shaping the rise and the fall of a burst and for controlling the power level of the burst.
A burst is the transmission quantum of numerous digital radio systems based on the principle of time division duplex (TDD), frequency division duplex (FDD), and code division duplex (CDD). The transmission takes place during a short time window. Within this time interval the emission rises from the starting power level to the nominal power level. The signal is then modulated to transmit a packet of bits. After that, the power level decreases until it reaches the minimum power level. The time mask of the burst, during which the bits are transmitted, is called a useful part or a payload part. Modulation is performed in the transmitter analogically or digitally, either at a base band frequency or at an intermediate frequency (IF). The modulated IF signal is then mixed up to the radio transmission frequency.
FIG. 1 depicts the rising portion of the power envelope of a burst comprising a digitally modulated intermediate frequency signal. In this example, during the rise of the burst the envelope should track a raised sine curve. After a predetermined period the power has reached its selected nominal level, whereupon modulation starts. This instant is denoted as 0 in the horizontal time axis. Usually the transmission power is adjustable according to the requirement of the system concerned. For that reason the nominal power level value can vary between the maximum power level and the minimum power level, as shown by the dotted line and the dashed line in FIG. 1. The nominal power level between those levels can usually attain one of several discrete power levels. For example, the downlink dynamic power control in the GSM system uses 16 power levels with 2 dB separations.
FIG. 2 depicts the falling portion of the envelope of a digitally modulated intermediate frequency signal. The signal envelope during the fall of the transmission burst should track a raised cosine curve.
The power level can be controlled burst by burst. Control is realized by scaling the ramp curve which follows the raised sine/cosine curve. Hence, the ramp-up curve starts from the minimum power level, but settles at the level specified by the power level indication as shown in FIG. 1. At the end of the burst the ramp-down curve starts from the nominal power level, but settles at the minimum power level as shown in FIG. 2.
Conventionally, power ramping and control of the output power level are performed in the analog domain. One problem with analog solutions is inaccuracies caused by aging and by variations in operation temperature and components. Furthermore, the analog solutions are complex, and stability is a problem.
Today the tendency is to perform power ramping and output power level control digitally in order to avoid the afore-mentioned problems. Some basic solutions are presented below.
FIG. 3 illustrates in broad outline the formation of a modulated IF signal into a shape as shown in FIG. 1 and FIG. 2. Data symbols arrive at digital modulator 31 that carries out modulation according to the modulation scheme of the system concerned. A ramp generator in block 32 generates the rising and falling edges according to the raised sine/cosine curve and a flat portion between the curves. Digital output signals from the ramp generator and the modulator are then converted to analog signals in digital-to-analog converters 33 and 36: For removing the high frequency sampling components the analog signals are then filtered in low pass filters 37 and 38, whereupon the analog modulated signal is multiplied in analog multiplier 35 by the analog ramp signal in order to smoothen out the rise and fall of the burst. The output from the multiplier is the analog modulated IF signal with ramped power.
FIG. 4 illustrates another digital modulator. Data symbols arrive at digital modulator 41, which carries out modulation according to the modulation scheme of the system concerned and produces I and Q signals. Said signals are then converted into analog signals by DA converters 43 and 44. For removing the high frequency sampling components the analog signals are filtered in low pass filters 410 and 411, whereupon both the analog I signal and the analog Q signal are transformed into an intermediate frequency by mixer 45 and mixer 46, accordingly. After mixing the sum of I signal and Q signal are added up in analog adder 47 to form the sum signal. A ramp generator in block 42 generates the ramp signal, i.e. the rising and falling edges according to the raised sine/cosine curve and the flat portion between them. The digital ramp signal is then converted into an analog signal in converter 48. The high frequency sampling components are filtered in low pass filter 412, whereupon the analog modulated sum signal is multiplied in multiplier 49 by the analog ramp signal in order to smooth out the rise and fall of the IF burst. The output from the multiplier is the analog modulated IF signal.
Common to both prior art solutions described above are the performance of both the modulation and the generation of the ramp signal digitally but conversion of the digital result signals into the analog domain before multiplying. However, there is a tendency in the art to carry out all processes within the digital domain. In order to better understand one possible realization of the digital ramp generator, a short review of the mathematical background is of assistance.
The burst signal can be considered as a product of an original modulated signal m(t) and a periodical switching signal sw(t). The spectrum of the burst signal is the convolution of the spectra of these two signals in the frequency domain.
For rectangular switching, i.e. without raised cosine/sine shaping, formula (1) is valid for frequency response:                               W          ⁡                      (            f            )                          =                                            M              ⁡                              (                                  f                  -                                      f                    C                                                  )                                      *                          Sw              ⁡                              (                f                )                                              =                      K            ⁢                                          ∑                                  n                  =                                      -                    ∞                                                  ∞                            ⁢                              xe2x80x83                            ⁢                                                M                  ⁡                                      (                                          f                      -                                              f                        C                                            -                                              nf                        g                                                              )                                                  ⁢                sin                ⁢                                  xe2x80x83                                ⁢                π                ⁢                                  xe2x80x83                                ⁢                                  nf                  g                                ⁢                                  τ                                      π                    ⁢                                          xe2x80x83                                        ⁢                                          nf                      g                                        ⁢                    τ                                                                                                          (        1        )            
where
* denotes convolution,
fc is the carrier frequency,
fg is the burst gating rate,
xcfx84 is the burst length and
K is a proportional constant.
For raised cosine/sine switching, i.e. with raised cosine/sine shaping, formula (2) is valid for frequency response:                               W          ⁡                      (            f            )                          =                  H          ⁢                                    ∑                              n                =                                  -                  ∞                                            ∞                        ⁢                          xe2x80x83                        ⁢                                          M                ⁡                                  (                                      f                    -                                          f                      C                                        -                                          nf                      g                                                        )                                            ⁢                              (                                  τ                  -                                      T                    r                                                  )                            ⁢              sin              ⁢                              xe2x80x83                            ⁢                              c                ⁡                                  (                                                            nf                      g                                        ⁡                                          (                                              τ                        -                                                  T                          r                                                                    )                                                        )                                            ⁢                                                cos                  ⁡                                      (                                          π                      ⁢                                              xe2x80x83                                            ⁢                                              T                        r                                            ⁢                                              nf                        g                                                              )                                                                    1                  -                                                            (                                              2                        ⁢                                                  T                          r                                                ⁢                                                  nf                          g                                                                    )                                        2                                                                                                          (        2        )            
where Tr indicates the ramp duration, and H is proportional constant.
The spectrum of the periodic burst signal consists of infinite numbers of secondary spectral lobes having the same shape as M(f) separated by the burst gating rate fg, and having decreasing amplitudes. Since the secondary spectral lobes resulting from formula (2) decay faster than those resulting from formula (1), the raised cosine/sine switching is used.
The following function is used to smooth out the rise of the burst:                                                         (                              A                -                dc                            )                        ⁢                                          sin                ⁡                                  (                                                            π                      ⁢                                              xe2x80x83                                            ⁢                      t                                                              2                      ⁢                                              T                        r                                                                              )                                            2                                +                      xe2x80x83                    ⁢          dc                ,                            (        3        )            
where
Tr indicates the ramp duration,
t is [0 Tr],
A is the envelope of the modulated signal, and
dc is the dc offset which settles the starting power level in FIG. 1.
The following function (4) is used to smooth out the fall of the burst:                                           (                          A              -              dc                        )                    ⁢                                    cos              ⁡                              (                                                      π                    ⁢                                          xe2x80x83                                        ⁢                    t                                                        2                    ⁢                                          T                      r                                                                      )                                      2                          +                  xe2x80x83                ⁢                  dc          .                                    (        4        )            
FIG. 5 illustrates a ramp generator and an output power controller known in the art which are based on formulas (3) and (4). The raised sine values of formula (3) or the raised cosine values of formula (4) are stored in the read only memory (ROM) 51. Digital multiplier 52 is used to control the amplitude level, i.e. value (Axe2x88x92dc). Adder 53 sets the dc offset, i.e. the last factor of formulas. The size of the ROM memory is about (fcfkxc3x97Tr)xc3x97outw, where fclk is the digital IF modulator clock frequency (sampling frequency), Tr is the pulse duration and outw is the multiplier input width.
One drawback of this known ramp generator is that due to the high clock frequency in the digital IF modulators, the size of the memory is large. For example, if the clock frequency is 52 MHz and the ramp duration is 14 xcexcs, then the size of the memory is about 728xc3x9712 bit. Furthermore, a multiplier is needed as an extra component to set the output power level.
Another possible way to implement the ramp generator and output power controller is to use a FlR-filter (Finite Impulse Response). The number of the FIR filter taps is fclkxc3x97Tr, where fclk is the digital IF modulator clock frequency and Tr is the pulse duration. One drawback of filter implementation is that due to the high clock frequency (sampling frequency) in the IF modulators, there are many taps in the FIR. Therefore, the realization of raised sine and cosine functions with filters is complex. For example, with the above mentioned values, i.e. the clock frequency is 52 MHz and the ramp duration is 14 xcexcs, the number of the FIR filter taps is 728.
One objective of the present invention is to devise a digital ramp generator with an output power controller that is easy to implement and which requires a minimum number of standard components, without the need for raised sine and cosine memories or digital filters.
A further objective is to devise a digital ramp generator with an output power controller generating a digital output signal that can directly multiply the digital modulated signal produced by a digital modulator.
Yet a further objective is to devise a digital ramp with inherent power control, wherein the generator and power control form a functionally inseparable integrated unit.
The present invention is based on a further mathematical explication of the raised sine and cosine curves representing the rise and fall of the burst. Simplifying functions representing raised sine/cosine curves to functions representing simple sine/cosine curves makes it possible to implement an electrically equivalent circuit consisting of a few simple basic components while tracking the raised sine and cosine functions well.
The core of the electrical equivalent circuit functioning as a ramp generator with an output power level controller is a second-order direct-form feedback structure forming a digital sinusoidal oscillator. The structure is well-known as such, and it produces an output sequence which is the sampled version of the pure sine wave with an amplitude value. The initial values of two state variables of the oscillator are chosen so that they both contain a predetermined first constant value. This first constant value will emerge as the amplitude value of the pure sine wave generated by the oscillator. Particularly the first constant value is equal to the desired nominal level A of the ramp minus a dc offset, where the dc determines the starting power level of the rising ramp and the settling power level after the falling ramp. The dc offset may also be called as a base level.
A second constant value equal to the desired nominal level of the ramp plus the above-mentioned dc offset is added to the oscillator output. Due to the deliberately chosen first and second constant values, the adding operation causes the rising ramp to start from level 2xc2x7dc and to end at level 2xc2x7A. Accordingly, the adding operation causes the falling ramp to start from level 2xc2x7A and to end at level 2xc2x7dc.
Finally, the result will be scaled so that the nominal power level will be A, and the starting level of the rising ramp and the end level of the falling ramp will be dc.
After the ramp has risen to the predetermined nominal power level, the output power level will be kept constant up to the instant when the ramp starts falling.
The proposed digital ramp generator and power controller can be implemented with the aid of two two-input adders, two delays, a multiplexer, and a fixed multiplier, which can be constructed with (Nxe2x88x921) adders, where N is the number of non-zero bits in the coefficient. The proposed ramp generator and output power controller saves hardware compared to the conventional methods. Furthermore, since the proposed ramp generator and output power level controller needs neither a memory nor a multiplier, it can be easily implemented with standard cells.